1. Field of the Invention
The present invention relates to a testing device for a squib resistance element and a method of testing a squib. More particularly, the invention relates to a circuit for testing the resistance of a resistance-type squib.
2. Background Information
A squib is a resistance element, such as a low-resistance wire coated with or embedded in an explosive material. Squibs are activated by applying a current to the resistance element. When a current flows through the resistance element, the resistance element is heated, and the resulting high temperature can ignite the explosive material. A high current applied to the squib will heat the resistance element rapidly, causing very fast activation, and a lower current can also activate the squib when applied to the squib for a longer period of time.
Squibs are used as ignitors in various systems, including for example, missiles and vehicle airbag systems. To ensure the squibs will ignite when an activating current is applied, the squib resistance is measured to verify that the squib resistance falls within a desired range. If the squib resistance is too low, the activating current may not generate enough heat to ignite the explosive material in the desired time. If the squib resistance is too high, the squib will be more likely to ignite if even a low current is applied, rendering the squib too sensitive to stray electrical fields.
Squib test and monitoring devices have been used for testing inflatable air bag systems and other squib-activated systems. U.S. Pat. No. 5,461,358, Ravas Jr. et al., and U.S. Pat. No. 5,825,282, Aronne, for example, disclose squib resistance testing devices which determine the value of the squib resistance. U.S. Pat. No. 5,166,880, Furui, U.S. Pat. No. 5,081,442, Ito et al., and U.S. Pat. No. 4,893,109, Vrabel et al., each disclose a fault detection device which employs a comparator circuit to periodically provide a good/bad indication for each squib in an airbag activating system. U.S. Pat. No. 5,268,643, Aso et al. describes a fault detection system which determines whether a short circuit has occurred in the squib.
U.S. Pat. No. 5,668,528, Kitao et al., and U.S. Pat. No. 5,805,058, Saito et al., disclose squib actuating circuits for use in vehicle passenger protection systems. The test system disclosed in U.S. Pat. No. 5,850,085, Rosenbluth, activates the squib in a vehicle airbag system and measures the deployment of the airbag.
Other airbag squib resistance test devices use a charge time comparator circuit to determine whether squibs have sufficient resistance. Examples of these devices are found in U.S. Pat. No. 5,293,153, Rochette et al., U.S. Pat. No. 5,541,523, Tourville, and U.S. Pat. No. 5,656,991, Hargenrader et al., the disclosures of which are incorporated herein in their entirety.
A fault detector for a squib activating circuit for an air bag system is disclosed in U.S. Pat. No. 5,677,838, Itou et al. The system prevents accidental activation of a squib if a computer fault occurs.
One method for non-destructively testing squibs in missile systems has been to apply a small current across the squib using a battery powered ignition tester. It is desirable that the current applied across the squib does not actually ignite the squib during testing. This is particularly important in missile system testing. Traditionally, it has been considered risky to use ignition testers whose current is drawn from the AC power source available at the facility. Concerns included faults in the ignition tester or in the facility power source which could allow excess current to be applied to the squib, or even that a lightning strike or other surge in the power lines could cause excess current to be applied to the squib. These events had the potential to cause the explosive material of the squib to ignite. To avoid these risks, battery powered squib testers have traditionally been used to test squibs in missile systems.
A battery powered ignition tester manufactured by Valhalla Scientific Inc. has been used for determining squib resistance in missile systems. Because the maximum current which this tester can provide is less than the current necessary to activate most squibs, this device was considered unlikely to activate the squib. The batteries in this ignition tester support only limited hours of operations before the batteries must be recharged. In full scale production facilities, the need to recharge the batteries can cause production delays and increased costs.
Other ignition testers are available which use a Wheatstone bridge to limit the current. One such device is the manually operated ignition tester manufactured by Alinco, which limits the current to 150 milliamperes and a low voltage.
The disclosures of each of the above identified patents are incorporated herein by reference in their entireties.